Hamas emerged as the Palestinian wing of Egypt's Muslim Brotherhood after the outbreak of the first intifada in 1987 and is the largest Palestinian militant organization. An Arabic word that means zeal or enthusiasm, Hamas is also an acronym for the group's official Arabic name, the Islamic Resistance Movement.

Its goal is to "liberate" Palestinian territories from Israeli occupation, and it has launched rockets and suicide bombers in pursuit of that end. The US, Israel, and the European Union consider it a terrorist group. But its military wing is not its only operation. Hamas also runs a large social services network and a political wing. In 2006, it participated in legislative elections for the first time and won a majority of seats, defeating Mahmoud Abbas's Fatah Party. International donors, which provide much of the Palestinian Authority's budget, cut off aid when Hamas refused to recognize Israel's right to exist.

In 2007, Hamas expelled Fatah from Gaza after a violent conflict, leaving the Palestinian territories divided, with Fatah controlling the West Bank.

What does Hamas believe?

Hamas says the land of Palestine is a God-given endowment to the Palestinians. Its long-term goal is the establishment of a Sunni Islamic state on all of historical Palestine. This vision leaves no room for Israel. Recently, however, leaders have indicated they'd accept a truce with Israel and the establishment of a Palestinian state on part of Palestine if Israel withdraws to its pre-1967 borders and Palestinian voters approve the move in a referendum.

"There are mixed messages," says Glenn Robinson, at the Naval Postgraduate School in Monterey, Calif. "If they say they're willing to accept a two-state solution ... but, at the same time, they say that they expect Israel to be destroyed in the future, and they themselves will never recognize Israel, that leaves room for people to say that this is kind of a temporary cynical thing until they gain the upper hand."

•Ismail Haniyeh is the prime minister of Hamas's government in Gaza and is considered more moderate than Mr. Meshal.

Tension has appeared at times between leaders in Gaza and the politburo in Damascus. Those on the ground in Gaza often take a more pragmatic stance than Damascus. During Israel's January offensive in Gaza, the internal leadership was ready to declare a cease-fire long before Damascus was, says Dr. Ranstorp. That conflict killed more than 1,300 Palestinians and 13 Israelis.

Some observers expressed hope after the 2006 polls that participating in government would have a moderating effect on Hamas. Instead, the events of the past few years appear to have bolstered hard-liners.

"Who prevails inside Hamas is almost entirely dependent on the conditions on the ground," says Dr. Robinson. "Since they won the January elections of 2006 – and especially since June of '07 – there's been an ongoing siege of Gaza, and those conditions favor the hard-liners."

Who funds Hamas?

Because Hamas controls Gaza, it can levy taxes and import duties on smuggled goods and raise municipal fees. It also receives donations from Muslim organizations around the world, for whom Gaza is an important cause. It has received substantial aid from Iran as well as from donors in Gulf states such as Saudi Arabia and Qatar.

What is Hamas's relationship with regional players?

Hamas is a nationalist movement, but that has not kept it from forming alliances or accepting aid – financial or military – from Iran, Hezbollah, Syria, and others. It has increasingly been pulled into Iran's orbit.

"For a long time, Hamas has resisted coming too close to the Iranian agenda," says Ranstorp. "Over time, Hamas has been forced to take money, given the hardship, and that has forced them closer to Iran."

Despite a religious and ethnic divide, Hamas and Iran unite over a common enemy: Israel.

But Hamas sticks to its nationalist agenda and has kept its distance from groups like Al Qaeda. "In its own rhetoric, Hamas tries to distinguish its moderation vis-à-vis groups which take a more global approach," says an international analyst in the region. "It doesn't see its cause as being global jihad."

How popular is Hamas?

Though Hamas won 74 of 132 parliamentary seats in the 2006 elections, it carried only about 45 percent of the popular vote. Robinson says that's a high-water mark for the organization. Much of its support comes from "derivative supporters – people who were sick and tired of Fatah were tired of a peace process that was all process and no peace," he says.

Fatah, riddled with corruption and led by an older generation that refuses to pass the torch, ceded much support to Hamas, which ran on an anticorruption platform of reform. Hamas is seen by many Palestinians as the only group standing up to Israel – and one that stepped into a vacuum to provide social services for the poor.

But the international analyst says support for Hamas in Gaza has trailed off as conditions have worsened, particularly after the war – though that hasn't meant more support for Fatah.

"There are some aspects which the people of Gaza do credit Hamas with achieving. Law and order is better than before Hamas won the election," he says. "But in terms of its ability to connect with the outside world, the population has paid a very high price. [Hamas has] achieved certain things, [but] they've failed to deliver many more. So their popularity is not nearly as high as it was."